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Ok, no problem! It does not matter. But thank you for the hint about ArcHydro. I will test it in the next days... Best Armin
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02-21-2013
07:24 AM
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Hello, oh yes sorry, I didn´t tell that I´m just use the Spatial Analyst tools in ArcGIS. I have not heard about the Arc Hydro tools yet. In fact the linkage of catchments is one specific fact, that is difficult to build with Spatial Analyst. Especially for my aim: I want to calculate the risk of erosion by the RUSLE-formula. Two of its factors deal with the flowlength (namely the maximum flowlength of a hillside) and the slope. For single hillsides it is no problem to identify both the flowlength and the slope (e.g. using an average slope). In a discontinuous area (DEM) it is quite more complicated. My idea was: - identify the flowpath (flowaccumulation) and calculate the flowlength...what is no problem - build watersheds by setting a threshhold for the flowaccumulation (e.g. 1000) and use the functions of pourpoints and watershed - the problematic step follows: following the flowpath inside a watershed the slope changes...possibly severely. When I want to calculate the erosion-risk I cannot use one characteristic slope-value of the whole watershed but have to divide it in more smaller parts because the RUSLE-formula enables the calculation of parts by using weighting-factors. - so my way is to build sub-watersheds relating to the changing in slope (e.g. when the slope of the following cell is more than twice or less than half the actual slope I will set a pourpoint for the sub-watershed) The point is: Is it possible to describe an inhomogious area by just one slope value? Or which variation requires a sub-division? Do you know if the ArcHydro tool provides some functions to approach a solution? (sub-dividing a watershed relating to slope?) Thank you for all your answers... Best Armin
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02-20-2013
11:13 PM
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Hello, sorry for my wrong term. What I meant is: during the flowdirection the cell is lifted up just virtually, so that it can get a flowdirection-value to its lowest neighbour. Of course the original elevation is not changed here. Thank you for the explanation of the rationale of the flowdirection...indeed it´s important to fill the wrong cells before the later analysis. I think I have to write more clear to say that my problems was to understand the "sink"-process. The only input there is a flowdirection-raster...and no information of the elevation is given to the process. Following the results of "sink", not just the "deepest cells" but also other cells are marked as "sinks". And even sometimes the deepest cells are neglected bein marked as "sinks", whereas the neighbour with the lowest height is a sink. Reading the helps in ArcGIS, I got the information, that the sink-tool just looks for either 1) undefinded flowdirections or 2) cells which flow into one another. Of course this makes sense. But it doesn´t take care of information of the actual elevation. Because of the name "sink" I would expect that the deepest cells would be marked as "sinks" as well. Can anyone explain these strange results? A second aspect is the "fill"-procedure. I know, why it is useful and an essential step in preprocessing to lift up error-cells. But I don´t get some information 1) which cells are lifted up and 2) which cell gives its elevation for the new and lifted value. In the ArcGIS-help relating to "sink" (http://help.arcgis.com/de/arcgisdesktop/10.0/help/index.html#/na/009z00000065000000/), it is described, that you can create a watershed based on "sink"-cells and look for the minimum value and its boundary. Every cell which is lower than this watershed-boundary is lifted up then. That is comprehensible! But in my DEM-examples I had two problems or disagreements: - sometimes the new value, to which the low cell are lifted up, lies beside (outside) the watershed? (I searched for the value in the original DEM and found it right beside the watershed based on the sink) So why do the swales get this value, when it is not a part of the watershed? Or in other words: How is the new value of the swales determined? It is definitely not just the next neighbour, what the help in ArcGIS hypothesizes (I tested it). - in some cases there was no "sink"-cells found, but a "fill"-process lifted up the deepest cells anyway. So is the question: Which point is chosen as the pourpoint for the watershed? In a short form my questions are: 1) Which cells are getting marked as "sinks" and why do not all deepest cells become a "sink"? 2) Which cells are lifted up in the "fill"-process (deepest cells or sink-marked ones)? 3) Which cell or value is responsible for the uplift-value (minimum in watershed?) and how can I determine it (when there are no sinks)? I hope on your answers! Thank you Armin
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02-15-2013
03:45 AM
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Hello, I´m writing my Master-thesis and have some problems to understand the results of "flowdirection" and "sink". When a cell is lower than all neighbours it is lifted up (during the flowdirection-process) to the height of the lowest neighbour. Normally the current cell and this neighbour will flow into one another. A result what is expected is e.g.: In the raster ...720 705 711 714... the 705 is lifted up to the next neighbour (711) and gets the flowdirection "1", whereas the 711 flows back and gets the "16". So both cell are marked as "sink". But in my opinion (and my results of DEM-preprocessing confirm my ideas) "sink" don´t really find cells, which are lower than their neighbours, what I would expect by the name "sink". Rather the output gives cells with an undefined flowdirection, which can appear because of several reasons. E.g. when the lowest cell is lifted up and the neighbour has another decisive flowdirection, the deepest cell won´t be marked as a "sink". ...720 705 711 708 700... (705 is lifted up to 711 and gets flowdir=1, but 711 has flowdirection=1 as well and doesn´t flow back to the lifted 705, because 708 is the decisive flowdirection. Is this right? Why the deepest value 705 is neglected in the "sink"-procedure? Another astonishing case is when two low cells have another cell between (e.g.: .....720 705 711 705 718...) the both 705-values get 711 in the flowdirection-process and get flowdirections "1" and "16" resp. The 711 gets the flowdirection=17 because of the same vertical height in two directions. The point I don´t understand: Why the two 705-points won´t marked as "sinks"; instead just 711 is a "sink" although it is not the deepest value in th neighbourhood? With this context the first example gives randomly the expected result (705=sink). I hope you can help me to better understand the functions "flowdirection" and "sink" or show me my mistake in theory! Best, Armin
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02-14-2013
06:48 AM
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Hello, i have two questions about the RUSLE-formula given above. 1. The L is calculated per cell by flowacc*resolution. What about the case ofdiagonal neighbourhood or the takeoff cells? Therefor the length must be 1,41*resolution and 0,5*resolution resp., isn´t it? 2. The flacc-value near the end of the flow (e.g. at the edge of the map) it´s quite high because of the accumulation. The formula says it must be multiplied by the S-Factor. But this is just the value just the last cell and not the average slope? I think you have to multiply the Slope and the Length per cell to a cel-LS-value and then make the sum of all to get the LS-value for a inclined area? Hope to get some information whether my concern is correct or some declaration why I err. Thank you!
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11-16-2012
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